Pascal Bourguignon <spam at thalassa.informatimago.com>:
> Even with sed I find difficult to do that.
If you want to remove blank lines, then sed '/^$/d' will do. If, on
the other hand, you actually want to compress pairs of newlines into
single newlines (as I think the specification may have said), something
a bit more complicated is required. I think that sed 'N;P;/\n$/d;D'
will do it.
If you want to use Perl or Ruby, these are equivalent to the first sed
solution:
ruby -ne 'print unless /^$/'
perl -ne 'print unless /^$/'
In Ruby you might also use:
ruby -pe 'next if /^$/'
The equivalent doesn't work in Perl, probably because -p puts the
implicit print statement in a "continue" block, which gets executed
before the loop is restarted by "next".
If you're doing this on a huge file and care about speed, sed will
be slightly faster than Perl and Perl will blow Ruby away. Because
I was curious, I tried writing it in C to see if I could beat sed.
My first attempt was slightly slower (!), but my second attempt ran
much faster than sed; unfortunately, it also took 30 minutes and
34 lines.
Jesse